Disc drive memory systems are widely utilized throughout the world today in traditional computing environments and more recently in additional environments. These systems are used by computers and more recently by devices including digital cameras, digital video recorders, laser printers, photo copiers, jukeboxes, video games and personal music players. Consequently, the demands on disc drive memory systems has intensified because of increased performance demands and due to new environments for usage. Disc drive memory systems store digital information that is recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the hub, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
Spindle motors have in the past used conventional ball bearings between the hub and the shaft. However, the demand for increased storage capacity and smaller disc drives has led to the read/write head being placed increasingly close to the disc surface. The close proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, possibly damaging the disc drive and resulting in loss of data. Conventional ball bearings exhibit shortcomings in regard to these concerns. Imperfections in the raceways and ball bearing spheres result in vibrations. Also, resistance to mechanical shock and vibration is poor in the case of ball bearings, because of low damping. Vibrations and mechanical shock can result in misalignment between data tracks and the read/write transducer. These shortcomings limit the data track density and overall performance of the disc drive system. Because this rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a spindle motor having fluid dynamic bearings on the shaft and a thrust plate to support a hub and the disc for rotation. One alternative bearing design is a hydrodynamic bearing.
In a hydrodynamic bearing, a lubricating fluid such as gas or liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Hydrodynamic bearings eliminate mechanical contact vibration problems experienced by ball bearing systems. Further, hydrodynamic bearings can be scaled to smaller sizes whereas ball bearings have smallness limitations. Dynamic pressure-generating grooves formed on a surface of the fixed member or the rotating member generate a localized area of high pressure and provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing and between the rotating surfaces, enabling the spindle to rotate with more accuracy. However, hydrodynamic bearings suffer from sensitivity to external loads or mechanical shock events. Fluid can in some cases be jarred out of the bearing by vibration or shock events. Further, bearing fluid is susceptible to evaporation over time. Further, bearing fluids can give off vaporous components that could diffuse into a disc chamber. This vapor can transport particles such as material abraded from bearings or other components. These particles can deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs.
Proper sealing is critical in the case of hydrodynamic bearings, and efforts have been made to address these problems. A capillary seal is typically employed to ensure fluid is maintained within a bearing. Here, a fluid meniscus is formed between two walls and capillary attraction retains the fluid.
Further, there is a trend to reduce the axial height of the fluid dynamic bearing motor since smaller profile disc drives are desired. However, as motors become shorter in height, the spacing between bearing components decreases, minimizing the angular or rocking stiffness of the bearings. It is important to maximize the available axial height for the bearings to support the relative rotation of the shaft and sleeve. The axial height of the capillary seal is therefore minimized to maximize the available axial height for the bearings. However, in minimizing the capillary seal height, the reservoir volume is reduced.
Recent designs employ a radial capillary seal having diverging walls wherein the diverging walls form a fluid reservoir for fluid lost due to evaporation. The capillary seal, being radial, minimizes the axial height of the capillary seal. Further, in a reservoir having larger volume, lower viscosity oil may be used, lowering power loss due to viscous friction. However, with a larger reservoir having diverging walls, the capillary seal gap is widened and thus the oil retention capability is lowered. Moreover, although a radial capillary provides some shock resistance, its shock resistance is limited. Fluid can be dislodged from a reservoir by shock, which moves a portion of fluid under a fluid fill hole included as part of the capillary seal. The fluid can potentially splash out during an initial shock event or during a subsequent shock event. Tests show that recent radial capillary seal designs fail at about 500 Gs of shock, and fluid leaks through fill holes at about 500 Gs of shock.
Mobile applications require higher resilience to shock events than desktop or enterprise products. Laptop or portable computers can be subjected to large magnitudes of mechanical shock as a result of handling. It has become essential in the industry to require disc drives to be capable of withstanding substantial mechanical shock. What is needed is a capillary sealing system that is axially minimized to maximize the height available for bearings. Further, a capillary seal system with a fluid reservoir that can withstand greater than 500 Gs shock is needed.